1. Field of the Invention
The present invention relates to a permanent magnet dynamo-electric machine, for example, a permanent magnet dynamo-electric machine such as a servo motor applied to an electric power steering device for a vehicle.
2. Description of the Related Art
A permanent magnet dynamo-electric machine has recently been employed for various purposes, and low cogging torque is required for its performance. Cogging torque causes vibrations. For example, in a motor used in an electric power steering device, cogging torque has to be greatly reduced to improve steering feelings.
To meet demands for miniaturization and low costs of a motor, a so-called “position sensorless driving” technique has been developed, which detects a rotational angle to drive the motor even without including any rotational angle detector such as an encoder or a resolver. For example, for this position sensorless driving, there is a technology where a nonmagnetic layer is disposed on an outer surface of a rotor in a section of electrical angles of 80 to 100° in forward and backward rotational directions of the rotor around a polar boundary separating N and S poles from each other (e.g., refer to Japanese Patent Application Laid-open H09-294391).
There is another technology which externally fits and fixes a cylindrical member (e.g., refer to Japanese Patent Application Laid-open No. 2006-109663).
However, the conventional art has the following problems.
In the above-mentioned a permanent magnet dynamo-electric machines, around the polar boundary separating the N and S poles from each other, in the section of the electrical angles of 80 to 100° in the forward and backward rotational directions of the rotor, the nonmagnetic layer is disposed on the outer surface of the rotor, or the cylindrical member is externally fitted and fixed. Thus, there is a problem of a small change in inductance made by the rotational angle.
In the case of the example of the conventionally known permanent magnet synchronous motor of the self-starting type, no change in inductance is made by the rotational angle because of improper disposition of the conductor, causing a problem of unsuitability for rotational angle detection.